Resin molded articles having a three-dimensional shape or complex shape are molded typically by injection molding. Injection molding can mass-produce molded articles having a desired shape. However, in order to manufacture molded articles that are required to have a high dimensional precision by injection molding, an expensive die having a high dimensional precision is required. Furthermore, since injection-molded articles are readily deformed by shrinkage and/or residual stress after the injection molding, the shape of the die needs to be adjusted precisely depending on the shape of the molded article and properties of the resin material. Since fraction defective is high in injection molding, product cost thereby is often high. Furthermore, injection molding of a molded article having a large thickness is difficult due to shrinkage and/or residual stress.
In order to obtain molded articles having a three-dimensional shape or complex shape, a method for molding a secondarily molded article having a desired shape, the method comprising: extruding and molding a resin material; producing a stock shape for machining (also referred to as “stock shape for cutting”) having a shape, such as a plate, round bar, pipe, special shape, or another shape; and subjecting the stock shape for machining to machining, such as cutting, drilling, and shearing, has been known. The method of machining the stock shape for machining has advantages, including that molded articles can be produced in small quantities at a relatively low cost because an expensive die is not required, that frequent modifications in molded article specifications can be accommodated, that molded articles with high dimensional precision can be obtained, that molded articles having a complex shape or large thickness, which is not suitable for production using injection molding, can be produced, and the like.
However, not all resin materials and/or extrusion molded articles are suitable as stock shapes for machining. A stock shape for machining needs to satisfy high levels of required properties, such as having a large thickness and excellent machinability, having low residual stress, being capable of avoiding excessive heat generation that leads to deformation and/or discoloration due to heat of friction generated during machining, being capable of being machined with high precision, and the like.
In general, most of processing methods used in metallic materials are utilized in machining of polymeric stock shapes as it is. Even among extrusion molded products, an extrusion molded product that is thin and has great flexibility, such as a typical film, sheet, or tube, is unsuitable for machining such as cutting. Even among extrusion molded products having shapes, such as plate or round bar, with a large thickness or large diameter, if the extrusion molded product has excessively large residual stress during extrusion molding, the extrusion molded product readily deforms during or after machining, and it is difficult to obtain a secondarily molded article having high dimensional precision. Even among extrusion molded products having reduced residual stress, the extrusion molded product that readily breaks or cracks during machining, such as cutting, drilling, and shearing, is not suitable as a stock shape for machining.
In order to obtain, via extrusion molding, a stock shape for machining having properties suitable for machining, selection of resin materials, method of extrusion molding, or the like needs to be devised. Therefore, various extrusion molding methods for producing extrusion molded articles suitable as stock shapes for machining, the method using resin materials that contain general-purpose resins and/or engineering plastics, have been proposed so far.
For example, Patent Document 1 discloses a method for producing a stock shape for machining having a thickness or diameter exceeding 3 mm, the method comprising solid-state extrusion molding a resin composition containing an engineering plastic such as a polyether ether ketone, polyetherimide, polyphenylene sulfide, polysulfone, polyether sulfone, or polycarbonate.
On the other hand, degradable plastics have drawn attention as polymer materials that have little adverse effect on environment, and have been used in applications including extrusion molded articles such as films and sheets, blow molded articles such as bottles, injection molded articles, and the like. Recently, application of biodegradable plastics in stock shapes for machining has been increasingly demanded.
Polylactic acid is known as a representative biodegradable plastic, and due to its moderate degradation rate, it is a biodegradable plastic that is preferably used depending on the application or usage environment. In addition, since polylactic acid is a polymeric material obtained by polymerizing a lactic acid obtained by fermenting a sugar taken from a plant-derived raw material such as corn, it has a carbon offset property that prevents increases in the amount of emission of CO2, which is a greenhouse gas circulating within the global environment, even when subjected to combustion treatment.
Incidentally, as the concern over resource constraints increases, there have been progressive increases in the depth and size of downholes (underground bore; hole provided to form a well such as an oil well or a gas well) provided to perform well drilling and completion to recover hydrocarbon resources from layers of the earth containing hydrocarbon resources (also simply called “petroleum” in the present invention) such as petroleum (such as shale oil) or gas (such as shale gas). For example, in horizontal boreholes formed laying almost horizontally in a shale reservoir or the like located below 1,000 m underground, a method of performing hydraulic fracturing (fracturing) has been widely used. Ball sealers for blocking a bore hole (fracture) formed by hydraulic fracturing and isolation plugs such as frac plugs, bridge plugs, packers, and cement retainers, which are downhole tools for well drilling and completion (also simply called “downhole tools” hereafter) installed in a downhole to perform hydraulic fracturing, are used to block locations near the tip of the downhole or locations where hydraulic fracturing was performed previously, and after hydraulic fracturing is newly performed again and a bore hole (fracture) is formed, these materials are recovered or broken. Therefore, downhole tool members for well drilling and completion (also simply called “downhole tool members” hereafter) provided on a downhole tool such as a ball sealer or an isolation plug are required to have strength capable of tolerating hydraulic fracturing or construction (for example, tensile strength) and to be cost-effective and easy to recover or break.
Typically, isolation plugs such as frac plugs, bridge plugs, packers, and cement retainers (also simply called “plugs” hereafter) have structures in which a rubber blocking member is attached around a plug core rod (also called “mandrel”), and the blocking mechanism of the isolation plug achieves a blocking effect by changing the shape of the rubber by tension and/or compression of the core rod (mandrel) (Patent Documents 2 and 3). The maximum size of the plug core rod (mandrel) is the inside diameter of the downhole, and the plug mandrel can have any predetermined outside diameter as long as a rubber blocking member can be attached around the plug mandrel. In many cases, the size of the plug mandrel is from 70 to 100 mm. Furthermore, the plug core rod (mandrel) often has a hollow shape in order to pass mud therethrough. The hollow diameter is, in many cases, from 10 to 50 mm, typically 19.1 mm (0.75 inches), 25.4 mm (1 inch), or 31.8 min (1.25 inches), and the mandrel has a shape comprising, for example, a main part having a pipe-like shape of approximately 1,000 mm in length, and a diameter expanded part at both ends so that a jig for performing tension and/or compression of the core rod (mandrel) can be engaged. During the tension and/or compression of the plug core rod (mandrel), a high load of approximately 1,500 to 5,000 kgf (approximately 14,700 to 49,000 N) and, in many cases, approximately 2,000 to 4,500 kgf (approximately 19,600 to 44,100 N) is applied to the core rod (mandrel). In particular, since 2 to 5 times the stress concentration is applied to the aforementioned diameter expanded part (engagement part with a jig) of the core rod (mandrel), a material having a strength that can tolerate such a high load must be selected.
After performing hydraulic fracturing, a method of retrieving the blocking member or breaking the core rod (mandrel) to form an opening is employed. Since metals such as cast iron have conventionally been used as the plug core rods (mandrels), the retrieval of isolation plugs involved high cost, and the breaking of the metal core rods (mandrels) also involved difficulty and high cost. Epoxy resin composite materials and the like have also been used as plug core rods (mandrels). However, even in resin composite materials such as epoxy resin composite materials, problems such as insufficient strength and the high cost required to retrieve the blocking member remain the same. In addition, there is a problem that, since resins and reinforced materials (such as carbon fibers and metal fibers) after breaking the core rods (mandrels) are non-degradable, it is practically impossible to treat and/or dispose of the resin composite materials.
In addition, relatively small ball sealers having a diameter of 16 to 32 mm and being formed from non-degradable materials such as aluminum or non-degradable plastics such as nylon or phenol resins coated as necessary with rubber to improve sealing properties have been conventionally used as ball sealers. However, in recent years, in step with increases in the depth and size of downholes, there is an increasing demand for ball sealers having an even greater diameter—for example, a diameter of 25 to 100 mm or an even greater diameter—and having a strength capable of tolerating a high load.
The use of degradable plastics as downhole tool members or ball sealers (also called “downhole tool members and the like” hereafter) is expected since degradable plastics can be disintegrated by leaving them in the downhole without recovering them to above ground after use. Specifically, there is a demand for a degradable plastic which has sufficient strength in environments exceeding 1,000 m underground (environments at temperatures exceeding 65° C. or the like), is capable of forming a downhole tool member or the like of a desired shape, and can decompose in environments at a variety of depths (that is, environments at a variety of temperatures), and a molded article thereof.
However, when a molded article of a downhole tool member or the like is produced with a general-purpose resin molding method such as injection molding, compression molding, or melt extrusion molding using a degradable plastic, most of which are crystalline resins, sink marks or voids are generated by thermal contraction after formation or shrinkage associated with crystallization, and the necessary dimensional precision cannot be achieved. Therefore, in order to obtain a downhole tool member or the like, attention has been focused on methods of performing machining such as cutting on a solid-state extrusion molded article with a large thickness or diameter formed by solidification and extrusion-molding from a degradable plastic.
The use of polylactic acid, which is a typical biodegradable plastic, in wells for oil drilling or the like is well known. Patent Document 4 discloses a viscous well treatment fluid containing polylactic acid, sand control screening or coating, and machinery disposed inside a well formed from polylactic acid or a part thereof, and a packer, a bridge plug, a cement retainer, or the like is disclosed as machinery.
It is further described in Patent Document 4 that the flexural strength of a rodlike body produced by injection molding from a crystalline poly-D-lactide (polylactic acid) is within the range of from 40 to 140 MPa and that a rodlike body formed by solidification and extrusion has a flexural strength up to 200 MPa, and this document references “Biomaterials 17 (March 1996, 529-535)” (Non-Patent Document 1). A rodlike body with a circular cross section produced by solidification and extrusion and consisting of a poly-D-lactide with a My (viscosity average molecular weight) of 160,000 is described in Non-Patent Document 1 as an “enhancement in the mechanical properties of polylactic acid by solidification and extrusion”, and mechanical properties such as the yield flexural strength of a solid-state extrusion molded article, which is a round barlike body with a diameter of 4 mm, is specifically disclosed.
It is not possible to form a downhole tool member or the like with a shape and size required by increases in the depth and size of downholes in recent years from the solid-state extrusion molded article, which is a round barlike body with a diameter of 4 mm, specifically disclosed in Non-Patent Document 1. In addition, since the glass transition temperature of polylactic acid is from 55 to 60° C., it is unclear whether a solid-state extrusion molded article of a rodlike body or the like formed from the poly-D-lactide disclosed in Patent Document 4 or Non-Patent Document 1 would be able to form a downhole tool member or the like with a desired shape having sufficient strength in environments exceeding 1,000 m underground (environments at temperatures exceeding 65° C. or the like) described above.